The development of catalysts with a diverse range of ligand frameworks to perform specific transformations has proven critical for both industry and academia. The success of both homogeneous and heterogeneous catalysis may be largely attributed to the use of various ligands to tune the behavior of transition-metal-containing catalyst complexes. Advances in the synthesis of transition-metal-containing complexes have in turn allowed for the development of novel industrial processes and the improvement of known processes in terms of scope, mildness and catalyst loadings.
For instance, transition metal complexes of cyclic alkyl amino carbenes have been found useful as catalysts for a range of applications (Bertrand et al., U.S. Pat. No. 7,312,331). Pd-cyclic alkyl amino carbenes complexes have been described by Bertrand and coworkers (Angew. Chem. Int. Ed. 2005, 44, 7236-7239) to be highly efficient catalysts for the alpha-arylation of ketones and aldehydes. Ru-cyclic alkyl amino carbenes complexes, which are have been used as catalysts for alkene metathesis processes, are also of great interest. In particular, a series of Ru-cyclic alkyl amino carbenes alkylidenes have been found to be highly active catalysts for the ethenolysis of methyl oleate as described in the Final Technical Report “Platform Chemicals from an Oilseed Biorefinery” (Award DE-FG36-04GO140016, Department of Energy funded joint project of Materia and Cargill).
Although this series of Ru-cyclic alkyl amino carbenes complexes demonstrates tremendous utility, the known synthetic routes to these Ru-cyclic alkyl amino carbenes alkylidenes are far from ideal, especially those routes to transition metal complexes of cyclic alkyl amino carbenes ligands having relatively small substituents such as 2,4,6-trimethylphenyl- or 2,6-diethylphenyl-bound to the nitrogen atom (Angew. Chem. Int. Ed. 2007, 46, 7262-7265). These synthetic routes usually include ligand substitution reactions which need to be conducted at sub-ambient temperatures. These reactions tend to be slow, typically resulting in poor yields and add to the cost of the synthetic route. Indeed, the desired Ru-cyclic alkyl amino carbenes complex is often obtained in poor yield. For example, although the above Final Technical Report identified complex 18 (shown in equation 5 of Angew. Chem. Int. Ed. 2007, 46, 7262-7265) as being the highest activity catalyst for methyl oleate ethenolysis, complex 18 was only obtained in an isolated yield of 18%. Accordingly, there is a need for improved synthetic routes to transition-metal-carbene complexes, in particular Ru-cyclic alkyl amino carbenes complexes, in order for these promising catalysts to be commercially viable.
The current invention relates to a carbene complex of a lithium and/or magnesium salt useful as a synthon in the preparation of transition-metal-carbene complexes. The current invention also relates to an improved synthetic route to transition-metal-carbene complexes by using a carbene complex of a lithium and/or magnesium salt as a synthon. More particularly, the present invention relates to an improved synthetic route to Ru-cyclic alkyl amino carbenes complexes using a carbene complex of a lithium and/or magnesium salt.
Advantageously, this improved synthetic route may be carried at temperatures at or above ambient, removing the need for coolants and thereby reducing the overall cost of the synthetic route. Even more advantageously, the improved synthetic route using the invention described herein provides transition-metal-carbene complexes in significantly higher yields than previous reported. Accordingly, the present invention provides a facile, mild, high-yield route to transition-metal-carbene complexes.